Programmed control system

ABSTRACT

A programmed welding system is described which automatically controls weld current, electrode travel rate, and weld duration time in an orbital type welder. Each program is carried on a single card bearing a number of resistors of which each controls one of the various parameters that is peculiar to the chosen program. A closed-loop modular transistorized power supply controlled by pulse width modulation is disclosed together with a combination of high frequency and impulse starting and precision pulse width modulation closed-loop control of electrode travel rate. All actions for a single weld operation are automatically controlled by the group of resistors on a selected program card. The operator, to perform a complete weld, need only select the desired program and operate the start switch.

United States Patent Burley et al. Sept. 5, 1972 [s4] PROGRAMMED CONTROLSYSTEM [72] Inventors: Richard Kenneth Burley; Robert Pfimary 'f f'Album) Friedman, both of Reseda; Howard Attorney--Wrlham R. Lane, ThomasS. MacDonald n. Losher, Canoga Park, all of Calif. and Allan Rothenberg[73] Assignee: 235th American Rockwell Corpora- 57 ABSTRACT A programmedwelding system is described which au- [22] Flled: March 1969 tomaticallyControls weld current, electrode travel 211 App] 04,251 rate, and weldduration time in an orbital type welder. Each program is carried on asingle card'bearing a l a number of resistors of which each controls oneof the a (5i. ..219/l31 various parameters that is peculiar to thechosen gram A closed loop modular transistorized power [58] Field ofSearch 2 321/18 96 4 supply controlled by pulse width modulation isdisclosed together with a combination of high frequency and impulsestarting and precision pulse width-modu- [56] References C'ted lationclosed-loop control of electrode travel rate. All UNITED STATES PATENTSactions for a single weld operation are automatically controlled by thegroup of resistors on a selected pro- 3,343,063 9/1967 Keeney, Jr. eta1. .....32l/l8 X gram cant The operator to perform a complete weld, ganeed only select the desired program and operate the U6 t t i h3,461,374 8/1969 Rhyne, Jr. ..321/l8 3,465,236 9/1969 James ..307/265 X16 Claims, 31 Drawing Figures 2222 Ills my SWITCH /r 320 SELECTORDRIVER. 0

2-1/8 I -120 26 ff Z2: 1.. PROGRAM "M5,? cums/w SWITCH I 312; may \W J320 -sw/rc/-/ 24 ore/van 300 J 320' sw/rcH DR! VER -30 j SWITCH H F ARCDRIVER \rf o STARTER INTEGRATE HOLD IIO WELD-HEAD H2 I MOTOR 1 PWMCURRENT --p- PRE- PURGE P ATENTEDSE P SIHTZ I 3.689.734

SHEET OEUF 13 ls! LEVEL CURRENT 50b UP SLOPE I 5 TIM 55OC UP SLOPE IRATE 1 2nd, LEVEL I 2nd LEVEL I /53/"' KI TIME 50f J 3rd LEVEL Tr 'KCURRENT 5509 F r 3rd LEVEL I CURRENT I 4m LEVEL I I TIME I DOWN $L0PEJ50! mg MOTOR SPEEDI CONTROL 501 DOWg Q LOI-"E 48 LEVEL, THIRD LEVELFOUR TH 1 53 PURGE IMPULSE FIXTURE DELAY 4 INVENTORS.

RICHARD K. BURLEY HOWARD D. LESHER BY ROBERT FRIEDMAN ATTORNEYP'ATENTEDSEP 51912 3.689.734

SHEET UBUF 13 I L F 1 l QQDQQQ INVENTORS. RICHARD K. BURLEY HOWARD 0.LESHER ROBERT FRIEDMAN 72 m; ATTORNEY mm? 98k 33 $3 5G3 PATENTEDSEP 5m3689.734 sum OSUF 13 1 R677? R679 "R682 F/ 6. /0b INVENTORS.

' R/CHA RD K. BURLEY HOWARD 0. LESHER B ROBERT FRIEDMAN ATTORNEYPATENTEDSEP 51912 sum mar 13 WU U U U a9 FIG. //a

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INVENTORS. RICHARD K. BURLEY HOWARD D. LES'HER ROBERT FRIEDMAN FIG. 15g

ATTORNEY SCRGOZ CRSOGB FIG. I20

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CR602A I NVENTORS RICHARD K. BURLEY HOWARD 0. LESHER BY ROBERT FRIEDMANATTORNEY 1 i u CONTROL SYSTEM BACKGROUND OF THE INVENTION Theincreasingly stringent requirements and specifications for weld controland precision, particularly in the aerospace field, have significantlyexpanded the need for automatic welding equipment in which variationsdue to human errors are eliminated. In addition the need for constantreproducibility of weld quality for a large variety of alloys andworkpieces requires that reliance upon human skill be minimized.However, such an automatic system must be relatively simple to operateso that semi-skilled technicians may utilize the system withoutaffecting the quality of the weld produced. Further such a system mustbe sufficiently versatile to allow use in a variety of weldingcircumstances so that the cost of welding operation is not significantlyincreased in proportion to the quality of 'weld obtained.

Previous attempts to provide solution to the problem ofautomatic weldinghave produced automatic curve followers, photo cell scanners such asshown in our prior copending application Ser. No. 628,743 for AU-TOMATIC WELDING SYSTEM, other highly sophisticated and complexprogrammed camming arrangements, and punched card computer controlledsystems. Such systems all fail to meet the present requirements ofsimplicity of operation with minimized possibility of operator errortogether with precision repeat'ability of desired weld program andsimplified operator control.-

SUMMARY OF THE INVENTION The present invention provides an automaticcontrol system in which all control elements andparameters are preciselycontrollable by an operator in the field who will merely select adesired program and initiate operation of the equipment. Principles ofthe present invention are illustrated in connection with a preferredembodiment thereof which is directed toward control of an orbit arcwelder. The system embodies a number of signal circuits each of which isarranged to generate an output signal that controls a different one ofthe parameters necessary for a total welding operation. Such parametersinclude a variety of current levels, a predetermined amount and rate ofcurrent upslope and downslope, timing of the various operations,electrode travel rate, and arc control and initiation. Each of theparameter controlling circuits is arranged to be controllable itself inlinear relation to a resistance forming part of such circuit. All suchresistances, of which each controls a given parameter circuit, arecollected and mounted together mechanically and electrically for quickconnection and disconnection to the circuit whereby any given group ofcontrolling resistors may be removed as a group and a different group,establishing a different set of parameters and thus a different program,may be substituted therefor. In a preferred embodiment a number ofresistor groups are all mounted in a program module stack and one groupis selectable at the control of an operator by means of a simpleswitching arrangement.

In a described embodiment the programmed welding power supply includes aplurality of switching transistors that are repetitively operablebetween substantially high conductions and substantially low conductionat a controllable duty cycle. Improved closedloop current regulating andless current ripple are provided by a unique pulse width control of dutycycle. One or more of the switching circuits may be connected ordisconnected to provide an incrementally varing or modular power supply.

For improved starting of a welding arc in accordance with the describedembodiment of this invention, there is provided a high frequency currentsource that is initiated together with actuation of a high impulsestarter. Means are provided to switch current from the impulse starterthrough the electrode and arc upon ionization of the gas around theelectrode.

Motor speed control for driving the electrode over the workpiece isprecision controlled, in accordance with the preferred. embodiment ofthe invention,

through the same resistor program card by use of a pulse width modulatorwhich energizes the motor winding with discrete driving pulses. A ratefeedback for comparison with a commanded motor speed signal is derivedfrom the back electromotive force of the motor at intervals betweendriving pulses. The feedback signal is compared with the command signal,the error is stored and subsequently caused to generate a pulse forenergizing the motor winding.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a preferredembodiment of a programmed welder constructed in accordance withprinciples of the present invention;

FIG. 2 illustrates a single program card carrying the resistors whichcontrol the various parameter signal control circuits;

FIG. 3- illustrates a program card stack showing, for purposes ofexposition, three cards, related switching, and circuits operated fromthe cards;

FIG. 4 is a timing chart illustrating one complete program cycle of weldcurrent;

FIGS. 5a, 5b, 5c and 5d illustrate relay logic which controls theprogrammer of the illustrated embodiment, with the relays showngenerally in order of chronological operation;

FIG. 6 illustrates a timing circuit for purge operation;

FIG. 7 illustrates the power switch impulse-module and high frequencymodule for driving the weld arc;

FIG. 8 shows the current sensing circuit of the described embodiment;

FIGS. 9a and 9b illustrate the current generation circuits includingconstant current generators, and slope and rate circuits of thedescribed embodiment;

FIGS. 10a and 10b illustrate the circuitry of the pulse DESCRIPTION OFTHE PREFERRED EMBODIMENT Throughout the drawings like reference numbersare employed to designate like parts.

As illustrated in FIG. l the programmed welder of the preferredembodiment basically comprises two systems, the first of which providescontrol of the weld head and the second of which provides speed controlof the motor which drives the weld head relative to the workpiece. Aswill be readily appreciated, the described system is applicable to avariety of different types of welding equipment and welding drivesystems. For purposes of exposition there is described a system that isspecifically arranged for operation of an orbit arc welder such as isdescribed in US. Pat. Nos. 3,194,936 and 3,238,347. As particularlydescribed in these patents, the welding arrangement includes a chamberfor containing inert welding gas. The chamber is arranged to be securedto and about the periphery of a pipe or tubular member to be welded.Mounted within the chamber for motion about the pipe under control of adrive motor 112 and suitable gearing is a welding electrode torch orweld head 110 FIG. 1).

In general, to perform a weld of this nature, the atmosphere about theelectrode is purged with inert gas, the arc is started by high frequencyor impulse, the main weld head current is brought up to a first level ata predetermined rate, the motor is started to initiate travel of theelectrode relative to the workpiece and the weld current is held at afirst desired level, generally for the duration of a complete traverseof the electrode about the periphery of the workpiece. If a secondtraverse of the electrode about the workpiece is required, the currentlevel may be reset at the end of the first traverse and electrodemovement is continued for a second full circle, at which time the weldcurrent is decreased at a predetermined rate. When the current hasdecreased to a desired value, the current is shut off, the motor isstopped to stop the electrode and a second purge of the gas isaccomplished.

Accordingly it will be seen that the variables to be controlled by theprogram include starting current, a variety of operating current levels,up and downslope rates, time of duration for the various current levels,

delay time to enable startup of the motor and, of course, motor speed.In accordance with the present invention, each of these variables may becontrolled by a resistor card included in a program card stack 1 14. Oneof the resistor cards of the stack may be selected by the operator bymeans of a selector-controller 116. The resistors on the selectedprogram card each controls a given one of several timer circuits 118,current level and rate command circuits 120 and motor speed commandcircuits 122, all of which are under control of a group of solid stateor relay logic 24.

Current level or rate, as the case may be for a particular portion ofthe selected program, is generated under control of the selected programcard by a command circuit 120 and fed to an error comparator 26 whichcompares the commanded current level with an actual current level sensedat the weld head by a conventional meter shunt in the weld current path.This meter shunt, as is well known, provides a feedback voltageproportional to the weld current.

The error signal from the comparator 26 is fed to a pulse widthmodulator 28 which provides a series of output pulses of generallysquare wave configuration, each having a duration directly proportionalto a commanded current level. The output of the pulse width modulator isfed to control each of a plurality of switch drivers, 30a, 30b, 30c, 30dand 30a, each of which individually controls the state of a bi-statedevice or transistorized switch 320 through 33c respectively. Eachswitch is arranged to connect a weld current supply of negative 40 voltsdo to the weld head. It will be readily appreciated that, with theillustrated arrangement, one or more of the switches may be connected ordisconnected from the circuit whereby the total weld current availableto the weld head may be varied in increments of, for example, 20 amperesfor each of the current switches. With this arrangement, power supplyfor welding with a variety of maximum ranges may be readily achieved.Thus the total weld current available is variable in major increments of20 amps, for example, by adding or decreasing a weld cur= rent switch 32and driver 30. In addition, after choice of any major increment, thecurrent is selectively and proportionately variable under control of thepulse width modulator and current level as commanded from the programmedcard.

Under control of the program logic 24 both an impulse starter 34 and ahigh frequency are starter 36 are initially energized to first provide ahigh frequency ionization of the inert gas surrounding the electrode,and thereafter; upon such ionization, to provide a high amplitude shortduration pulse of current through the electrode to initiate the arc. Theare is thereafter sustained by the commanded weld current through thegroup of modular switches 32.

Starting and stopping of the motor 112 is under control of logic 24.Motor speed control is established by one of the resistors on theprogram card to provide a commanded preselected motor speed. In thedescribed arrangement the motor is driven by a plurality of pulses froma pulse width modulator 38 which is analogous to the pulse widthmodulator 28 of the power switch drivers. Pulse width modulator 38provides to the motor a repetitive series of pulses of varying dutycycle wherein the duty cycle commanded by the program card resistorrepresents the desired motor speed. In order to assure attainment of thecommanded speed, a rate feedback is employed in which, shortly aftertermination of each motor driving pulse from the pulse width modulator,the back electromotive force of the motor is sampled and fed via aswitch 40 as one input to an error comparison circuit 42. The errorcircuit receives as its second input a signal representative of thecommanded motor speed, which is fed to the comparator 42 by means of aswitch 44. Switches 40 and 44, which operate in effect as samplingcircuits, momentarily couple the rate feed-back signal and commandedspeed signal to the comparator which provides a difference signal to anintegrate and hold circuit 46 which stores the signal for subsequenttransmission as a pulse width modulator control signal. The samplingswitches 40 and 44 are ganged and operated in synchronism from the pulsewidth modulator, whereby the sampling and generation of the rate errorsignal is achieved at a predetermined interval subsequent to terminationof each driving pulse from the pulse width modulator 38. The motor speedcontrol circuit will be described in greater detail hereinafter inconnection with FIGS. 13, 14 and 15.

An exemplary program card, as illustrated in FIG. 2 comprises a rigid orsemi-rigid self-supporting card or support 48 on which are mounted agroup of resistors 50a through 50! inclusive. Resistors 50a through 50kall have one end thereof connected to each other and also connected incommon to a contact 22 of the program card. The commonly connected endsof resistors 50a through 50k are also connected in common to a parallelresistance capacitance circuit 52 which in turn is connected to a secondcontact 21 of the resistance card. The resistance card 48 includes aplurality of individual contacts indicated as contacts 1 through 22inclusive, of which contacts 3, 4, 5 and 6 are electrically nected atone end thereof to one side of a second resistance capacitance circuit56 and to contact 1 of the resistance card. The other end of resistor50! is connected by means of a diode 58, which is poled oppositely withrespect to the diodes connecting the other resistors of the card, tocard contact 2. The end of the resistance capacitance circuit 56 remotefrom its connection to resistor 50! is connected to card contact 21. Thereason for the unique treatment of resistor 501 lies in the fact thatthis resistor controls the downslope rate and must provide a negativevoltage to its control circuit whereas all of the other resistors arearranged to provide positive voltage to the circuits controlled thereby,as will be described below. Accordingly program card contact 1 isarranged to be connected to a negative volt d.c. supply whereas programcard contact 22 is arranged to be connected to a positive 24 volt d.c.supply as will be more particularly explained in connection with FIG. 3.

PROGRAMMING In a preferred embodiment of the program card stack, all ofthe cards and all of the resistances thereon are made identical and eachresistance is made variable comprising, for example, a potentiometer.Each such variable resistance is then set to provide the selectedresistance value of a given program. With such an arrangement anyprogram, once identified by the resistance settings, may be readilyduplicated at a different location for use in another similarlycalibrated machine. Values of the different resistors need be onlytransmitted from one location to another and an identically calibratedmachine at such second location to another and an identically calibratedmachine at such second location can thus be supplied with the programdefined and proven at the first location.

Illustrated in FIG. 3 is a program card stack embodying three programresistor cards 48, 60 and 62, each of which is identical to thatillustrated in FIG. 2 although as will be immediately apparent, thevalues of one or more of the resistors will be chosen or varied asnecessary or desirable to obtain a particular preselected program ofoperation. Although but three resistor program cards are illustrated inFIG. 3, and although such cards are employed in one actual operatingembodiment of the described invention, it will be readily appreciatedthat other numbers of cards may be provided or groups of cards may bereplaced collectively so that no limit on the total number of programsavailable'exists.

The program card selector comprises a group of banks or ganged switchesof which the banks are illustrated as comprising switches 64, 66 and 68,shown in position of the switch banks wherein the program carddesignated as 60 is selected for control. In the uppermost of the threeswitch positions, program card 48 is selected whereas in the lowermostposition the program card 62 is selected. It will be readily appreciatedthat the ganged switches 64, 66 and 68 may be manually operated withtheir operating positions visible to the operator. If larger numbers ofcards are employed, an automatic stepping switch of conventional designmay be employed so that the operator, to select a specified program,need only dial to an indicated card number to cause the switch banks tostep. in synchronism to the selected card. In the illustratedarrangement, resistors of all program cards are at all times coupled tothe circuits controlled thereby, to the extent permitted by the systemlogic. It is the connection of these resistors to the supply voltagethat is varied by operation of switches 64, 66, 68.

Switch 64 is connected to the negative 15 volt d.c. source mentionedabove and connects to contact 1 of the selected program card. Contact lis coupled via downslope rate resistor 501 and diode 58 to contact 2 ofthe program cards and then is connected through a set of normally openrelay contacts CR607B of the system logic to a downslope rate generator69 to be more particularly described hereinafter.

As previously mentioned, the logic of this program system employs aseries of solid-state relays and relay contacts. For ease inunderstanding the description, it is noted at this point that with thedescriptive terminology employed, the relay number which is prefaced byCR (control relay) indicates the relay energizing coil whennot followedby a letter suffix. When the relay number is followed by a lettersuffix, the indication is of a set of relay contact operated by therelay coil bearing the same number. Thus, for example CR607 denotes acontrol relay coil and CR607B denotes a set of contacts controlled byCR607 Card selector switch 66 couples the card through a normally closedstop push-button PB302 to a source of 24 volts d.c. via normally closedrelay contacts CR613A and CR608A.

Program card selector switch 68 is connected to a source of positive 24volt d.c. and couples to the contact 22 of the selected resistanceprogram card which is in turn connected in parallel to one side of eachof program resistors 500 through 50k inclusive. Program card contacts 21are all connected to a common line or ground as indicated.

Program card contacts 7 are connected to each other and then directly tothe motor speed control circuit 22, more particularly describedhereinafter in connection with FIGS. 13, 14 and 15. Program cardselector switch 66 connected as previously. described to a positive 28volt d.c. supply, is coupled via program card contacts numbers 3 through6 to a start-stop relay coil CR601 which is in turn connected to groundthrough parallel paths including a normally open start-push button PB301and normally open self-locking contacts CR601A which lock in relay coilCR601 when the latter is energized by operation of the start buttonPB301.

It will be noted that although like contact numbers, such as contactnumbers 7 through 17, for example, of the program card resistors, areall connected to each other, only one of the cards is operable for anyselected program since only one of the cards has the other end of itsresistor, namely that coupled with card contact number 22, connected tothe positive 24 volt d.c. supply.

Program card contacts number 8 are connected to provide downslope timingto a downslope timer 70, more particularly described hereinafter.Program card contacts 9 are connected to a fourth level timer 71.Contacts number 10 are connected to provide fourth level current commandby normally open relay contacts CR605B to the current level generator20. Contacts 11 are connected to a third level timer 72. Contacts 12 areconnected to provide third level command to the current level generator20 via normally open contacts CR603C and normally closed contactsCR605A. Program card contacts 13 are connected to a second level timer73. Program card contacts number 14 are connected to provide secondlevel current command to the current level generator via normally closedrelay contacts CR603D and normally open relay contacts CR606F. Programcard contacts 15 are connected to an upslope rate generator 74 vianormally closed relay contact CR605C. Program card contacts 16 areconnected to an upslope timer 75 and program card contacts 17 areconnected to provide to current level generator 20 the first levelcommand signal via normally closed relay contacts CR606E. The varioustimer, rate and current circuits 69 through 75 all will be describedwith greater particularity hereinafter.

Illustrated in FIG. is a graph of current level against time for atypical program under control of the embodiment of the inventiondescribed herein. It will be seen that prior to pressing the startbutton, upon providing power to the circuits, a pre-purge cycle isachieved by the purge timer to be described hereinafter. Then, uponpressing the start button, a first purge is accomplished, also undercontrol of the purge timer. At the end of the purge timing cycle, theimpulse start together with high frequency ionization occurs and weldcurrent is applied at a commanded upslope rate to attain a predeterminedor second level of weld current At the end of the upslope period a delayperiod is provided for fixture delay and thereafter the motor is startedand current to the weld head is established at a third level. Uponcompletion of the third level time period, normally coinciding withcompletion of one complete traverse around the periphery of theworkpiece, a second traverse may be started at a fourth weld currentlevel. It will be readily appreciated, however, that where but a singletraverse of the workpiece is required, the third and fourth level timeintervals may be decreased accordingly and the third and fourth currentlevels may be made equal. At the end of the fourth level time downslopeof the current level is initiated at a predetermined rate. Thereafterall is stopped, either by termination of the downslope timer interval orby decrease of the weld head current below a predetermined level.

Illustrated in FIGS. 5a, 5b, 5c and 5d is the relay logic of theillustrated embodiment of programmed welder which will be employed todescribe the sequence of operation of the various circuits that arecaused to operate under control of the program cards. The ensuingdescription will follow an exemplary program from its beginning to thecompletion of the weld with detailed explanation of the various circuitsas they come into operation.

On initial connection of power to the system a separate positive 28 voltd.c. source is energized and a purge solenoid 101 FIG. 5a) is energizedfrom the 28 volt dc. to ground via normally closed contacts CR609A toinitiate a pre-purge. At the same time the purge timer shown in detailin FIG. 6 initiates its timing operation. This is achieved by the logicshown in the upper portion of FIG. 5a providing 28 volt do to the purgetimer 76 or, more specifically, to the collector of transistor Q601thereof (FIG. 6) via normally closed relay contacts CR608A and CR613A,closed stop switch P8302 and closed start switch PB301B.

PURGE TIMER The purge timer FIG. 6) comprises a relaxation oscillatorformed by a pair of transistors Q601 and Q602 which are connected toform a constant current generator. Transistor Q601 acts as a zenordiode, holding the base voltage of Q602 constant to thereby provide aconstant current through the emitter-collector circuit of Q602. Thistransistor thus conducts via the path illustrated in FIG. 5a to charge atiming capacitor C625. The junction between the collector of Q602 andthe capacitor is connected to the control electrode of a uni-junctiontransistor 0603 which has the base electrodes thereofresistance-connected between the 28V supply and ground. When the voltageon capacitor C625 rises sufficiently, the uni-junction transistor 0603fires to provide a positive'pulse at TPl. This point is coupled to thecontrol electrode of a silicon control rectifier SCR601 which isconnected between ground and the collector of transistor Q602 by meansof a diode D601 and a resistor R604. When SCR601 fires, at the end ofthe purge timer interval, CR609 is connected through the SCR to groundand is therefore picked up (energized) as illustrated in FIG. 5a. WhenCR609 is picked up, contacts CR609A open to deenergize the purgesolenoid 101. Now the start button PBSOIA is pressed, opening PB301B totake the power off CR609 and extinguishing SCR601. Upon pressing startbutton PB301A relay CR601, the start-stop relay, is picked up viacontacts CR608A, CR613A and closed stop button PB302 as illustrated inFIG. 3. CR601 is locked in by locking contacts CR601A. This relay bymeans of contacts CR601D keeps the purge solenoid energized to maintainflow of inert gas to and about the arc.

When CR601 picks up, switched 28 volt d.c. is made available to thevarious control relay coils by means of contacts CR601B, and, by openingcontacts CR601E as shown in FIG. 9a, the first level command signal ismade available to the current command circuit.

At the end of this purge time interval, CR609 again picks up and, sincepositive 28 volt supply is now available through CR601, a relay coilCR610 is picked up via contact CR609A which may be. provided with a timedelay closure. CR610 locks up through locking contacts CR610A.

As illustrated in FIG. d, closure of contacts CR610B and CR610C provides120 volt a.c. current to a main contactor relay coil MC 101. The maincontactor relay, when picked up, provides the welding power supplycurrent as will be more particularly described hereinafter in connectionwith FIG. 8.

ARC STARTING Contacts CR610D also pick up a relay CR612, the impulserelay. Theimpulse relay, via contacts CR612A provides 120 volt a.c. tothe starter impulse circuit (see FIG. 7). This a.c. current is fedthrough a transformer T801 and a diode bridge BR801 to start thecharging of an impulse capacitor C801 via a resistor R801. A highfrequency start relay coil CR801 is connected across the impulsecapacitor C801 and, when the charge on the capacitor rises to asufficient level, CR801 picks up, closing contacts CR801A, and providing120 volt a.c. to a transformer T901 of the high frequency startercircuit. The high frequency starter circuit includes a standard RF sparkgap SP901 connected across a high frequency tank circuit comprisingcapacitor C901 and a very low d.c. resistance coil L901. The oscillatorysign in the tank circuit is applied across the welding torch. Thiscauses the gas between the torch electrode and the workpiece to ionize.The gas thereupon drops in impedance. Capacitor C801 is also coupled tothe torch via diode D801, inductance L701, resistor R707, and inductanceL901. With the low impedance between the torch and the workpiece due togas ioniza tion, capacitor C801 discharges through the described circuitto provide a high current short duration spike through the ionized gapwhich properly heats the electrode so that the arc may be maintained bycurrent from the main supply. When capacitor C801 discharges, relayCR801 drops out and the high frequency circuit is de-energized. If thearc is made, the current sense relay, to be described below inconnection with FIG. 8, picks up and turns off the 120 volt a.c. totransformer T801. If the arc does not start capacitor C801 recharges andthe starting cycle repeats.

CURRENT SENSE The current sensing circuit, FIG. 8, detects current onthe primary side of a three phase bridge which drives the power switchmodules. A source of welding current supply 80 such as three phase 440volt a.c. is fed through main contactor relay contacts 101a, b and cand, via a power transformer T101, to a bridge rectifier 81. At theoutput of this rectifier is provided the minus 40 volt d.c. supply tothe power switches 32 illustrated in block form in FIG. 1. A pluralityof sensing coils, 104, 105, 106, one for each phase, is coupled via aresistance network 82 to a low current bridge rectifier 83 of which theoutput is fed through a potentiometer 84 and zenor diode 85 to the baseof a transistor 0619 which has its emitter connected to ground andincludes a relay coil CR611 in its collector circuit. Capacitor C639 isprovided to prevent 0619 from turning on due to noise spikes and falsestarts. Accordingly, it will be seen that when an arc is initiated bythe impulse and high frequency starter circuitry previously described,the arc current is sensed by pickup coils 104, and 106, to provide asignal via the diode bridge 83 to the base of 0619, which thereuponpicks up the current sense relay CR611. This relay, as will be presentlydescribed, provides the rest of the system with the information that thearc has, in fact, been started. 7

Returning now to the relay logic circuit of FIG. 5b, it will be seenthat an on-off relay CR606 is picked up by contacts CR611A and locked inby contacts 606D. This on-off relay initiates a number of differentoperations including the dropping out of CR612, the impulse start relay,start of the upslope timer, pickup of slope relay CR614 to start theupslope rate, and switching of command current to the second level.

If CR606 picks up and the arc should be extinguished or the current fallto an unacceptable level whereby CR611, the current sensing relay, isdropped out, the logic provides for the pickup of an automatic resetrelay CR613 via contacts CR606D when contacts CR611B are in theirnormally closed position. If CR613, the automatic reset relay, shouldpick up, contacts CR613A, illustrated in FIG. 5a, will open to drop outthe start-stop relay CR601.

When on-ofi relay CR606 picks up, the high frequency impulse relay,CR612, drops out by opening of contacts CR606A whereby CR801 (FIG. 7)drops out and the high frequency is de-energized.

One of the functions achieved by the pickup of the on-off relay, CR606,is the switching of the commanded current from first level to secondlevel. This may be more readily understood in connection with thedetailed circuit diagram of FIG. 9.

CURRENT COMMAND Current level command is provided to the current levelgenerator 20 as illustrated in FIG. 3 via the programmed resistor and,for first level current command, via relay contacts CR606E (which areclosed before CR606 is picked up). Thus 24 volt d.c. supply is fed tothe current level generator via the first level program resistor whichthen controls the level of current to be generated. Current from thesupply is fed via the program resistor through resistors R655 andpotentiometer R656 of FIG. 9 to the collector of a transistor 0621. Asdescribed above in connection with a similar circuit of the purge timerof the FIG. 6, the base of the transistor Q621 is in effect clamped tol5V by a transistor Q620 whereby the circuit becomes a constant currentgenerator. A potentiometer R658 in the emitter circuit of 0621 enablesmanual control of the current to provide proper scaling. PotentiometerR656 is employed to furnish voltage level adjustment. A diode D623 isconnected between ground and the collector of 0621 to prevent this pointfrom going negative. Thus it will be seen that the collector voltage of0621 is a function of the program resistor. It will be recalled thatupon pickup of CR601, contacts CR601E are opened to remove the shuntformed by these contacts between the collector of Q62] and ground. Thevoltage at the collector of Q62! is therefore linearly related to theprogram card resistance 50a of FIG. 2. The collector of 0621 drives twostages of emitter followers comprising transistors 0622 and 0625, eachof which has a constant current generator connected between its emitterand ground. The constant current generator for 0622 comprises a pair oftransistors, 0624 and 0623, which are connected and which operate inmuch the same manner as transistors 0621 and 0620. Similarly theconstant current generator for 0625 is comprised of transistors 0627 and0626. These constant current generators minimize variation of currentloads with voltage level and thereby reduce effects of changing base toemitter voltage drops due to changing currents. The emitter of 0625drives a third emitter follower stage 0628 which has its collectorconnected to ground via a diode D625 and a resistor R665. The use ofthree stages of emitter follower circuitry enables conversion from therelatively high impedance source of 0621 to a low impedance sourcerepresented by 0628. This low impedance is required because it isemployed as a voltage clamp circuit as will be described hereinafter.

FIGS. 90 and 9b form a single circuit when FIG. 9b is locatedhorizontally aligned with and to the right of FIG. 9a. With relay CR614not yet pulled in, contacts CR6l4A (FIG. 9b) are in their normallyclosed position and the voltage on the emitter of 0628 is fed throughtwo stages of emitter follower circuits comprising transistors 0633 and0634. The signal at the emitter of 0634, at TPll forms the currentcommand generator output. This signal is fed tothe pulse width modulatorof FIG. 10 as more particularly described below.

The voltage at TPll may be monitored to facilitate adjustment ofpotentiometers R656 and R658.

Turning to the logical and chronological description, it will berecalled that upon pickup of CR606, the commanded current is changedfrom first level to second level. This may be seen in FIG. 3 whereincontacts CR606E opens and contacts CR606F close, thereby switching fromprogram resistor 50a to program resistor 50d which now controls thevoltage on 0621 of the current generating circuit. This second levelcurrent is not a current that is applied to command the weld current butis primarily employed to provide a level to which the command currentmay charge to at the upslope rate. Thus CR606, when it picks up, alsostarts the upslope timer and the upslope rate control.

From the resistor 50c of the program card and via normally closedcontacts CR605C, transistor 0630 (FIG. 9), connected as a constantcurrent generator with 0629, is caused to provide a current at a ratedetermined by the program resistor. Current flowing through 0630 iscaused to linearly charge a capacitor C640 which is connected betweenground and normally open contacts CR614B and to the collector of 0630.The slope of the increasing charge on capacitor C640 is controlled bythe amount of current provided to the capacitor via 0630. Apotentiometer R672 in the emitter circuit of 0630 provides for a scalingadjustment that is set up during initial machine calibration. After theupslope has been initiated, switching from level one current to leveltwo current, the rising voltage on capacitor C640 continues to increaseuntil it is clamped to the voltage level on TP5 through diode D628. Thisclamping level is a function of the preselected programming resistance.In this instance and at this time this is the second level currentcommand resistance.

With the pickup of relay CR614, via closing of contacts CR606D, contactsCR614A open and contacts CR614B close. Accordingly the voltage level onthe capacitor C640 is now the voltage that is fed via emitter followerstages 0633 and 0634 to the current command generator output, and,further, the level command voltage appearing on the emitter of 0628 isno longer coupled to the current command generator output. Thus thecircuit is under control of the upslope rate signal on capacitor C640and is not under any current level control.

It is convenient at this point, although not in chronological sequence,to describe the downslope portion of the circuit which operates in afashion similar to the upslope portion except that a negative ramp iscreated. This negative ramp is initiated at the completion of level fourweld time at whichtime, as will be described. hereinafter, CR605 ispicked up to cause rapid discharge of capacitor C640 through CR607A(FIG. 9b)'and now closed contacts CR605D (FIG. 9a) to the fourth levelweld of the current command voltage at the emitter 0628. Simultaneouslythis pickup of CR605 opens the emitter circuit of 0630 (opening contactsCR605C) so that when the downslope relay CR607 is picked up, only thetransistor 0632 of the downslope rate command will be connected acrossthe capacitor C640. The transistor 0632 is connected as a constantcurrent generator as previously described and. has its base clamped bymeans of a transistor 0631. Thus 0632 is a constant current generatorjust as previously described except that it is an NPN transistor ratherthan PNP and has its emitter tied via CR607B and a potentiometer R673 tothe source of negative 15 volt do. by means of the program card resistor50!, the downslope rate program resistor see also FIG. 3). Thisarrangement provides a linear discharge path for capacitor C640 whichcreates the downslope. Scaling is accomplished as before, in the samemanner as for upslope, where R673 is now the scaling resistor. It may benoted that the level on capacitor'C640 cannot drop below about 0.7 voltsdue to the clamping action provided by a diode D629 that is connected toa +24V through a resistor R667 and via diode D628 and R665 (FIG. 921) toground. Diodes D630 and D631 act as a zenor diode obtaining currentthrough R667 and providing the clamped voltage for D629.

TIMERS Once again referring to the description of the logic andchronology of a single program, it will be recalled that upon pickup ofCR606 operation of the upslope timer was initiated. This timer, togetherwith the second level timer, are shown in FIGS. 12a and 12b.

The program resistor controlled voltage to the upslope timer is providedvia program resistor 50b and resistor R607 through a potentiometer R608to the emitter of a transistor 0605 which has its base clamped bytransistor 0604 connected between +24 and ground, whereby the circuithas a constant current generator just as described in connection withthe purge timer of FIG. 6. However, in this instance, the amount ofcurrent provided by this constant current generator is controlled by theprogram resistor. The constant current provided by transistor 0605 isemployed to linearly charge a timing capacitor C627 which thereupon

1. A pulse width modulator for providing a train of output pulses at aselected repetition rate and with a selected duty cycle, comprising: acontrol signal; means for generating a repetitive ramp signal; acomparator responsive to the control signal and to the ramp signal forproducing an output pulse that is initiated when said ramp signalattains a magnitude having a predetermined relation to the controlsignal and that terminates upon termination of the ramp signal; a sourceof welding current; a plurality of switches arranged to connect anddisconnect the current source to and from a weld head; and meansresponsive to the output of said comparator for repetitively operatingsaid switches at a duty cycle represented by said control signal.
 2. Aprogramable control system including means to regulate the Currentsupplied to a welding torch, said apparatus comprising: a welding torch,a source of welding supply current, a source of program voltage, aplurality of signal circuits for generating output signals forapplication in controlling functions of said welding torch includingcurrent command generating circuits and timing circuits operable togenerate current command output signals and first and second timingoutput signals respectively, each of said current command generating andtiming circuits having therein a resistor which provides electricalcommunication between said current command generating and timingcircuits and said source of program voltage, each of said timingcircuits controlling a time interval having a value dependent upon thevalue of its said resistor; a logic circuit in electrical communicationwith said signal circuits including a first output signal operable tocontrol the operation of said timing circuits and a second output signalresponsive to said first output signal of said timing circuits operableto control said current command generating circuits; an error comparatorfor generating an output signal; means responsive to the second outputsignal of said timing circuits and to said second output signal of saidlogic circuits for generating a signal for input to said errorcomparator; a group of modulator transistor switches for providingelectrical communication between said source of welding supply currentand said welding torch; a pulse width modulator responsive to the outputsignal of said error comparator and operable to control the operation ofsaid switches and thereby control the current supplied to the weldingtorch; means for providing as a second input to said error comparator, asignal representing welding current.
 3. The control system of claim 2wherein one of said signal circuits includes a motor speed controlcircuit and further including: a motor for driving a workpiece relativeto the welding torch; a pulse width modulator for energizing the motorwith a series of motor driving pulses; means for deriving a motor speedfeedback signal during intervals between said motor driving pulses;means for sampling the feedback signal and the signal generated by themotor speed command signal circuit; means for comparing said sampledsignals and storing the comparison; means for disabling the lastmentioned pulse width modulator during the sampling time; and means forcontrolling the last mentioned pulse width modulator in accordance withthe stored comparison signals.
 4. A program control system according toclaim 3 including means for starting a welding arc, said starting meanscomprising: a high frequency starter for ionizing inert welding gas anda high frequency impulse starter responsive to ionization of said gasfor providing a high current pulse through the ionized gas and throughthe welding arc.
 5. A variable signal generating circuit comprising: alevel control circuit for generating level signals having predeterminedvalues; a capacitor for generating a varying signal; first constantcurrent means in electrical communication with said capacitor andoperable to charge said capacitor to the level of one of said levelsignals in electrical communication with said capacitor; means forrapidly charging said capacitor to the level of a second one of saidlevel signals; second constant current means in electrical communicationwith said capacitor for discharging said capacitor; an output terminal;logic means for selectively coupling the first and second constantcurrent generating means to the level control circuit and to the meansfor rapidly charging, said logic means including means for selectivelycoupling to said output terminal selected ones of said level signals andsaid varying signal; a plurality of groups of circuit elements; aprogram control means for selecting one of said groups; switch means forselectively connecting different ones of the elements of the selectedgroup into said level control circuit to determine the value of thesignal generated thereby; a welding torch; a current supply for thetorch; a power switch for selectively connecting the torch with thecurrent supply; a pulse width modulator responsive to signals on saidoutput terminal for operating said power switch.
 6. A welding powersupply comprising: a source of welding current; a plurality oftransistors each detachably connected in series between the currentsource and a welding electrode, said transistors being connected inparallel with each other; and means for repetitively switching at leastsome of said transistors together between a first state of substantiallyfull conduction and a second state of substantially no conduction,whereby the current to the welding electrode may be varied in incrementsby connecting a selective combination consisting of at least one of saidtransistors, and said transistors, when connected, operate only in a lowvoltage-high current state or high voltage-low current state to minimizepower dissipation.
 7. The power supply of claim 6 including means forvarying the time in which said transistors are maintained in said firststate, whereby the current to the welding electrode may be varied inaddition to or in lieu of said incremental variation.
 8. The powersupply of claim 6 including: closed loop control means for generating anerror signal indicative of the difference between a commanded currentand current drawn by said welding electrode; and said means forrepetitively switching comprising means responsive to the closed loopcontrol means for operating the transistors in accordance with saiderror signal.
 9. The power supply of claim 6 wherein said means forrepetitively switching comprises: a pulse width modulator connected tocontrol the duty cycle of said transistors; and closed loop controlmeans for selectively controlling the duration of pulses produced by thepulse width modulator.
 10. The welding supply of claim 6 including: aninductor in series with said transistors to minimize fluctuation of therepetitively switched current fed to the electrode; means to provide afeedback signal representative of weld current; a command signal; acomparator for generating an error signal indicative of the differencebetween said command and feedback signals; and a pulse width modulatorresponsive to said error signal for controlling the time during whichsaid transistors are maintained in said first state.
 11. A welding powersupply comprising: a current supply; a welding torch; and means forvarying current from the supply to the torch in predetermined discreteincrements and linearly with respect to a commanded level within saidincrements, said varying means comprising: a plurality of bi-statedevices detachably connected between said supply and torch, and a pulsewidth modulator connected to operate all of said devices with a dutycycle in accordance with said commanded level.
 12. The power supply ofclaim 11 wherein the pulse width modulator comprises: an oscillator; acapacitor; a constant current generator connected to linearly charge thecapacitor; means responsive to the oscillator for repetitivelydischarging the capacitor; a source of command signal; a modulatorcomparator having first and second inputs thereof connected to receive asignal from said capacitor and an error signal; a weld current circuitproviding a feedback signal representative of welding current, saidcircuit having included therein a resistor; an error comparatorresponsive to said feedback and command signals for generating saiderror signal; whereby the modulator comparator provides an output pulsethat is initiated upon attainment of a predetermined relation betweenthe inputs to the modulator compaRator and that is terminated upondischarge of the capacitor by said oscillator; and means for driving allof said devices in accordance with the output pulses from said modulatorcomparator.
 13. A modular power supply comprising: a plurality of powersupply units; a welding torch; and means for detachably connecting acombination consisting of at least one of said power supply units inparallel to said torch; each said unit comprising: a power switch, aninductor connected between the switch and the unit output, and a switchdriving circuit for operating the switch in response to a currentcommand signal; and command means for applying a common current commandsignal to all of said driving circuits in unison.
 14. The power supplyof claim 13 wherein said command means comprises: an oscillator; acapacitor in electrical communication with said oscillator; a constantcurrent generator connected to linearly charge the capacitor; a sourceof command signal; a modulator comparator having first and second inputsthereof connected to receive a signal from said capacitor and an errorsignal; a resistor in electrical communication with the current suppliedto said welding torch for generating a feedback signal indicative ofwelding current; an error comparator responsive to said feedback andcommand signals for generating said error signal; whereby the modulatorcomparator provides an output pulse that is initiated upon attainment ofa predetermined relation between the inputs to the modulator comparatorand that is terminated upon discharge of the capacitor by saidoscillator; and means for controlling said driving circuits inaccordance with the output pulses from said modulator comparator.
 15. Aclosed loop welding power supply comprising: a source of weldingcurrent; a welding torch; a switch electrically interposed between thesource of welding current and the torch; means for generating a feedbacksignal indicative of current through the torch; a source of currentcommand signal; and means responsive to said feedback and currentcommand signals for repetitively operating said switch between states ofconduction and nonconduction at a controlled duty cycle.
 16. The powersupply of claim 15 wherein said source provides direct current, saidlast mentioned means comprises a comparator for generating an errorsignal indicative of the difference between the current command andfeedback signals, a pulse width modulator responsive to the comparatorfor producing a train of pulses, each having a duration in accordancewith said error signal, and means for operating the switch in accordancewith the pulses of said train of pulses.